Ceramic piezoelectric transformer was first introduced by C. A. Rosen in 1956. Its operation principle is different from the magnetic transformer that transforms electromagnetic energy. The piezoelectric transformer first transforms electromotive force to mechanical energy (this process is called inverse piezoelectric effec), then transforms the mechanical energy to electric energy (this process is called positive piezoelectric effect). The ceramic transformer is a new type of electronic transformer made by sintering ferroelectric ceramic material and polarized under high pressure. Its basic operation frequency ranges from dozens KHz to hundreds KHz in the supersonic frequency zone.
The piezoelectric transformer makes fabricating high efficiency and small size converters possible. Compared with the piezoelectric transformer, the magnetic transformer of the same given power is thicker, heavier and has a lower efficiency. But its cost is lower, and can operate in a wide range of loading conditions. Hence for a long time the cold cathode fluorescent lamp (CCFL) that requires high actuating and ignition voltage is supported by the winding magnetic transformer with a high coil ratio. However, development of the piezoelectric transformer has great progresses in recent years. Now it can offer advantages such as higher efficiency, smaller size, lower electromagnetic noise, higher ignition voltage, non-flammable, and sinusoid operation, etc. For instance, R.O.C. patent publication No. 504101 discloses a “High luminosity fluorescent lamp driving apparatus” which employs the piezoelectric transformer.
The piezoelectric transformer includes a rectangular piezoelectric ceramic layer to perform energy transformation, a pair of primary electrodes (for input) and a pair of secondary electrodes (for output). Electric signals input to the primary electrodes are converted to mechanical vibrations through a piezoelectric fashion. The mechanical vibrations are transferred to the secondary ceramic layer to be converted to a high electric potential through the piezoelectric fashion to be output. The voltage gain of the piezoelectric transformer is the function of piezoelectric material coefficient g( ), primary layer number, material thickness and the entire length, as indicated in the equation below:V (Voltage gain)=(Length×Layer No./Thickness)g( )
In terms of the hardware structure, the piezoelectric transformer has single sheet (or single layer) structure and laminated structure. The single sheet piezoelectric transformer may be fabricated easier at a lower cost, but has a lower voltage gain (typically 5–10), and might require a voltage boosting magnetic transformer to actuate a lamp set. On the other hand, the laminated piezoelectric transformer has a higher voltage gain (20–70) and can actuate a load of a greater power. For instance, R.O.C. patent publication No. 492204 entitled “High output laminated piezoelectric transformer” is such an example. But the design and fabrication of the laminated piezoelectric transformer is more difficult, and the production yield is lower. At present only a few well known manufacturers have the fabrication techniques for such products. Moreover, the laminated piezoelectric transformer to actuate CCFL requires different output powers depending on varying CCFL specifications of various vendors. The manufacturers have to customize the laminated piezoelectric transformer to meet customer's requirements. Hence fabrication cost is higher, and the specification is not very flexible.